1. Field of the Invention
The present invention relates to a target employable for three-dimensionally measuring a position and an attitude occupied by an object such as a space craft or the like relative to a space station or other space craft when the first-mentioned space craft performs a docking operation with a space station or with another space craft.
Further, the present invention relates to a system for enabling a position and an attitude occupied by an object such as a space craft or the like to be three-dimensionally measured from a measuring point remote from the object by using the aforementioned target.
2. Description of the Related Art
To facilitate understanding of the present invention, a conventional target and a conventional system of the aforementioned type will be briefly described below with reference to FIGS. 9-11.
FIG. 9 is a perspective view which schematically shows the structure of a conventional target employable for three-dimensionally measuring a position and an attitude occupied by an object such as a space craft or the like wherein the conventional target has been disclosed in pages 51-54, B1-3, 1988, Symposium on Artificial Intelligence, Robotics and Automation in Space Application. FIG. 10 is an illustrative view which schematically shows the structure of a conventional system for three-dimensionally measuring a position and attitude of an object such as a space craft or the like wherein the conventional system has been disclosed in the same symposium as mentioned above.
In FIG. 9, reference numeral 1 designates a plane plate, reference numerals 2a, 2b, 2c and 2d designate a plurality of marks which are arranged on the plane plate 1 while exhibiting a rectangular shape, respectively, reference numeral 41 designates an intersection where two diagonal lines extending between the marks 2a, 2b, 2c and 2d intersect each other, reference numeral 42 designates a pole which is upright in relation to the intersection 41 on the plane plate 1, and reference numeral 43 designates a mark which is fixedly mounted on the top of the pole 42. A target 9 employable for three-dimensionally measuring a position and attitude of an object such as a space craft or the like is constituted by the above-described components.
Next, in FIG. 10, reference numeral 8 designates an object to be measured, reference numeral 9 designates a target attached to the object 9 for enabling three-dimensional measurement of a position and attitude occupied by the object 8, reference numerals 10a, 10b and 10c designate a coordinate axis in an imaginary target coordinate system which is arranged on the target 9, respectively, reference numeral 11 designates a measuring point, reference numeral 12 designates an imaginary reference coordinate system which is arranged on the measuring point 11, reference numerals 12a, 12b and 12c designate a coordinate axis which constitutes the reference coordinate system 12, respectively, reference numeral 44 designates a TV camera which is installed on the measuring point 11, reference numeral 45 designates a synchronizing signal separating circuit for separating a synchronizing signal from an output from the TV camera 44, reference numeral 46 designates a counter circuit for allowing each output from the synchronizing signal separating circuit 45 to be input thereinto, reference numeral 47 designates a buffer memory in which an output from the counter circuit 46 is temporarily stored, and reference numeral 26 designates a calculating/processing circuit which performs a calculating/processing operation in accordance with the program which has been previously determined with reference to the content of items stored in the buffer memory 47.
Next, an operation of the conventional system constructed in the aforementioned manner will be described below.
When the target 9 attached to the object 8 is visually recognized from the measuring point 11 via the TV camera 44, an image as shown in FIG. 11 can be obtained.
In FIG. 11, reference numeral 20a designates an image portion corresponding to the mark 2a on the plane plate 1, reference numeral 20b designates an image portion corresponding to the mark 2b on the plane plate 1, reference numeral 20c designates an image portion corresponding to the mark 2c on the plane plate 1, reference numeral 20d designates an image portion corresponding to the mark 2d on the plane plate 1, reference numeral 48 designates an image portion corresponding to the mark 43 and reference numeral 49 designates a point corresponding to the intersection 41.
When an output from the TV camera 44 is input into the synchronizing signal separating circuit 45, an image signal is separated from the synchronizing signal. In response to the image signal, a mark detecting circuit (not shown) detects points each having a high degree of brightness, i.e., the image portions 20a, 20b, 20c and 20d which represent the marks 2a, 2b, 2c and 2d on the plane plate 1. Then, the counter circuit 46 calculates a position of each of the detected image portions 20a, 20b, 20c and 20d in the image as shown in FIG. 11 by utilizing the synchronizing signals which have been generated in the synchronizing signal separating circuit 45.
Next, a principle for operating the conventional system will be described below.
Each synchronizing signal generated in the synchronizing signal separating circuit 45 is composed of a vertical synchronizing signal and a horizontal synchronizing signal. With such a construction, the position occupied by each detected mark in the image as seen in the horizontal direction can be calculated by counting the time which has elapsed from receipt of the horizontal synchronizing signals in the counter circuit 46. On the other hand, the position of each detected mark as seen in the vertical direction can likewise be calculated by counting the number of horizontal synchronizing signals which have been received by the counter circuit 46 in synchronization with the vertical synchronizing signals. Values derived from counting operations performed in the counter circuit 46 are stored in the buffer memory 47. As is apparent from FIG. 10, the calculating/processing circuit 26 is arranged upstream of the buffer memory 47 so as to have access to the buffer memory 47. Now, the position and the attitude occupied by the object 8 can be calculated in three dimensions in accordance with the software which has been previously programmed by using values derived from the foregoing access.
Next, a principle for three-dimensionally measuring the position and the attitude occupied by the object 8 relative to the measuring point 11 with reference to the image in FIG. 11 which has been visually recognized by the TV camera 44 will be described below.
The position and the attitude of the object 8 relative to the measuring point 11 can be determined by three components representing the positions of three coordinate axes 10a, 10b, 10c and 10d in the target coordinate system relative to the reference coordinate system 12 and three components representing the attitude angles of the same.
Generally, in a case where the geometrical positional relationship among four points on a single common plane in the three-dimensional space is previously known, it has been clarified that the three-dimensional positions corresponding to the four points can definitely be determined by an inverse perspective transformation process (refer to a thesis titled "A few considerations on inverse transformation associated with perspective transformation" by Shimazaki in the articles collected by Image Engineering Division of Japanese Electronic Communication Society, 79-15, 1979). In addition, a system for three-dimensionally measuring a position and an attitude occupied by an object based on the aforementioned principle while four marks arranged on the apex of a rectangular form are assumed as a target, respectively, has already been reported (refer to a thesis title "Application to three-dimensional position/attitude sensors and robots" by Ishii et al. in the articles collected by Japanese Measurement and Automatic Control Society, Vol. 21, No. 4, 1985).
As is apparent from the reference to the above-cited thesis, with the conventional system constructed in the above-described manner, the position and the attitude occupied by the axes 10a, 10b and 10c in the target coordinate system relative to the reference coordinate system 12 can be determined and the position and the attitude occupied by an object to be measured can be three-dimensionally measured by calculating the gravity centers corresponding to the image portions 20a, 20b, 20c and 20d in the image as shown in FIG. 11. This is because the image portions 20a, 20b, 20c and 20d in the image representing the target 9 which has been visually recognized by the TV camera 44 in the reference coordinate system 12 correspond to points derived from perspective transformation of the marks 2a, 2b, 2c and 2d on the plane plate 1. Further, since a length of the image portion 48 corresponding to the mark 43 mounted on the apex of the pole 42 and a length of the image portion representing the point 49 in correspondence to the intersection 41 vary with an excellent sensibility in response to variation of the attitude angle of the object 8, the conventional system can measure the attitude angle of the object 8 with a high degree of accuracy by utilizing the aforementioned properties of the conventional system.
However, since the conventional target is constructed in the above-described manner, a length of the pole 42 must be elongated in order to measure the attitude angle of the object 8 with improved accuracy. This leads to a problem that a measuring operation cannot practically be performed because the mark may be hidden due to extension of the pole 42 or the pole 42 may collide with another space craft when a docking operation is being performed. On the contrary, when the length of the pole 42 is reduced so as to obviate the aforementioned problem, there arises another problem that the attitude angle of the object 8 cannot be measured with a sufficiently high degree of accuracy.